Review of TCO Thin Films.docx
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Review of TCO Thin Films.docx
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ReviewofTCOThinFilms
ReviewofTCOThinFilms
Abstract
Thepresentreviewpaperreportsonthephysicalproperties,status,prospectsforfurtherdevelopment,andapplicationsofpolycrystallineoramorphous,transparent,andconductingoxides(TCO)semiconductors.Thecoexistenceofelectricalconductivityandopticaltransparencyinthesematerialsdependsonthenature,number,andatomicarrangementsofmetalcationsincrystallineoramorphousoxidestructures,ontheresidentmorphology,andonthepresenceofintrinsicorintentionallyintroduceddefects.TheimportantTCOsemiconductorsareimpurity-dopedZnO,In2O3,SnO2andCdO,aswellastheternarycompoundsZn2SnO4,ZnSnO3,Zn2In2O5,Zn3In2O6,In2SnO4,CdSnO3,andmulti-componentoxidesconsistingofcombinationsofZnO,In2O3andSnO2.SndopedIn2O3(ITO)andFdopedSnO2TCOthinfilmsarethepreferablematerialsformostpresentapplications.TheexpandinguseofTCOmaterials,especiallyfortheproductionoftransparentelectrodesforoptoelectronicdeviceapplications,isendangeredbythescarcityandhighpriceofIn.ThissituationdrivesthesearchforalternativeTCOmaterialstoreplaceITO.TheelectricalresistivityofthenovelTCOmaterialsshouldbe~10-5Ω.cm,typicalabsorptioncoefficientsmallerthan104cm-1inthenearUVandvisiblerange,withopticalbandgap~3eV.Atpresent,ZnO:
AlandZnO:
Ga(AZOandGZO)semiconductorscouldbecomegoodalternativestoITOforthin-filmtransparentelectrodeapplications.ThebestcandidatesareAZOthinfilms,whichhavelowresistivityoftheorderof10−4Ω.cm,inexpensivesourcematerials,andarenon-toxic.However,developmentoflargeareadepositiontechniquesarestillneededtoenabletheproductionofAZOandGZOfilmsonlargeareasubstrateswithahighdepositionrate.Inadditiontotherequiredelectricalandopticalcharacteristics,appliedTCOmaterialsshouldbestableinhostileenvironmentcontainingacidicandalkalisolutions,oxidizingandreducingatmospheres,andelevatedtemperature.MostoftheTCOmaterialsaren-typesemiconductors,butp-typeTCOmaterialsareresearchedanddeveloped.SuchTCOinclude:
ZnO:
Mg,ZnO:
N,IZO,NiO,NiO:
Li,CuAlO2,Cu2SrO2,andCuGaO2thinfilms.Atpresent,thesematerialshavenotyetfoundplaceinactualapplications.
I.Introduction
Mostopticallytransparentandelectricallyconductingoxides(TCO)arebinaryorternarycompounds,containingoneortwometallicelements.Theirresistivitycouldbeaslowas10-4Ω.cm,andtheirextinctioncoefficientkintheopticalvisiblerange(VIS)couldbelowerthan0.0001,owingtotheirwideopticalbandgap(Eg)thatcouldbegreaterthan3eV.Thisremarkablecombinationofconductivityandtransparencyisusuallyimpossibleinintrinsicstoichiometricoxides;however,itisachievedbyproducingthemwithanon-stoichiometriccompositionorbyintroducingappropriatedopants.Badeker(1907)discoveredthatthinCdOfilmspossesssuchcharacteristics.Later,itwasrecognizedthatthinfilmsofZnO,SnO2,In2O3andtheiralloyswerealsoTCOs.Dopingtheseoxidesresultedinimprovedelectricalconductivitywithoutdegradingtheiropticaltransmission.AldopedZnO(AZO),tindopedIn2O3,(ITO)andantimonyorfluorinedopedSnO2(ATOandFTO),areamongthemostutilizedTCOthinfilmsinmoderntechnology.Inparticular,ITOisusedextensively.
TheactualandpotentialapplicationsofTCOthinfilmsinclude:
(1)transparentelectrodesforflatpaneldisplays
(2)transparentelectrodesforphotovoltaiccells,(3)lowemissivitywindows,(4)windowdefrosters,(5)transparentthinfilmstransistors,(6) lightemittingdiodes,and(7) semiconductorlasers.AstheusefulnessofTCOthinfilmsdependsonboththeiropticalandelectricalproperties,bothparametersshouldbeconsideredtogetherwithenvironmentalstability,abrasionresistance,electronworkfunction,andcompatibilitywithsubstrateandothercomponentsofagivendevice,asappropriatefortheapplication.TheavailabilityoftherawmaterialsandtheeconomicsofthedepositionmethodarealsosignificantfactorsinchoosingthemostappropriateTCOmaterial.TheselectiondecisionisgenerallymadebymaximizingthefunctioningoftheTCOthinfilmbyconsideringallrelevantparameters,andminimizingtheexpenses.TCOmaterialselectiononlybasedonmaximizingtheconductivityandthetransparencycanbefaulty.
Recently,thescarcityandhighpriceofIndiumneededforITO,themostpopularTCO,asspurredR&Daimedatfindingasubstitute.Itselectricalresistivity(ρ)shouldbe~10-4Ω.cmorless,withanabsorptioncoefficient(α)smallerthan104cm-1inthenear-UVandVISrange,andwithanopticalbandgap>3eV.A100nmthickfilmTCOfilmwiththesevaluesforαandρwillhaveopticaltransmission(T)90%andasheetresistance(RS)10Ω.Atpresent,AZOandZnO:
Ga(GZO)semiconductorsarepromisingalternativestoITOforthin-filmtransparentelectrodeapplications.ThebestcandidatesisAZO,whichcanhavealowresistivity,e.g.ontheorderof10−4 Ω.cm,anditssourcematerialsareinexpensiveandnon-toxic.However,thedevelopmentoflargearea,highratedepositiontechniquesisneeded.
AnotherobjectiveoftherecentefforttodevelopnovelTCOmaterialsistodepositp-typeTCOfilms.MostoftheTCOmaterialsaren-typesemiconductors,butp-typeTCOmaterialsarerequiredforthedevelopmentofsolidlasers.Suchp-typeTCOsinclude:
ZnO:
Mg,ZnO:
N,ZnO:
In,NiO,NiO:
Li,CuAlO2,Cu2SrO2,andCuGaO2thinfilms.Thesematerialshavenotyetfoundaplaceinactualapplications.
PublishedreviewsonTCOsreportedexhaustivelyonthedepositionanddiagnostictechniques,onfilmcharacteristics,andexpectedapplications.,,Thepresentpaperhasthreeobjectives:
(1) toreviewthetheoreticalandexperimentaleffortstoexplorenovelTCOmaterialsintendedtoimprovetheTCOperformance,
(2)toexplaintheintrinsicphysicallimitationsthataffectthedevelopmentofanalternativeTCOwithpropertiesequivalenttothoseofITO,and(3) toreviewthepracticalandindustrialapplicationsofexistingTCOthinfilms.
II.Electricalconductivity
TCOsarewidebandgap(Eg)semiconductingoxides,withconductivityσintherange102–1.2106(S).Theconductivityisduetodopingeitherbyoxygenvacanciesorbyextrinsicdopants.Intheabsenceofdoping,theseoxidesbecomeverygoodinsulators,withρ>1010Ω-cm.MostoftheTCOsaren-typesemiconductors.Theelectricalconductivityofn-typeTCOthinfilmsdependsontheelectrondensityintheconductionbandandontheirmobility:
σ=μne,whereμistheelectronmobility,nisitsdensity,andeistheelectroncharge.Themobilityisgivenby:
whereτisthemeantimebetweencollisions,andm*istheeffectiveelectronmass.However,asnandτarenegativelycorrelated,themagnitudeofμislimited.Duetothelargeenergygap(Eg>3 eV)separatingthevalencebandfromtheconductingband,theconductionbandcannotbethermallypopulatedatroomtemperature(kT~0.03eV,wherekisBoltzmann’sconstant),hence,stoichiometriccrystallineTCOsaregoodinsulators.ToexplaintheTCOcharacteristics,variouspopulationmechanismsandseveralmodelsdescribingtheelectronmobilitywereproposed.Somecharacteristicsofthemobilityandtheprocessesbywhichtheconductionbandispopulatedwithelectronswereshowntobeinterconnectedbyelectronicstructurestudies,e.g.,thatthemobilityisproportionaltothemagnitudeofthebandgap.
Inthecaseofintrinsicmaterials,thedensityofconductingelectronshasoftenbeenattributedtothepresenceofunintentionallyintroduceddonorcenters,usuallyidentifiedasmetallicinterstitialsoroxygenvacanciesthatproducedshallowdonororimpuritystateslocatedclosetotheconductionband.Theexcessordonorelectronsarethermallyionizedatroomtemperature,andmoveintothehostconductionband.However,experimentshavebeeninconclusiveastowhichofthepossibledopantswasthepredominantdonor.Extrinsicdopantshaveanimportantroleinpopulatingtheconductionband,andsomeofthemhavebeenunintentionallyintroduce.Thus,ithasbeenconjecturedinthecaseofZnOthatinterstitialhydrogen,intheH+donorstate,couldberesponsibleforthepresenceofcarrierelectrons.InthecaseofSnO2,theimportantroleofinterstitialSninpopulatingtheconductingband,inadditiontothatofoxygenvacancies,wasconclusivelysupportedbyfirst-principlecalculationsofKiliçandZunger.TheyshowedthatSninterstitialsandOvacancies,whichdominatedthedefectstructureofSnO2duetothemultivalenceofSn,explainedthenaturalnonstoichiometryofthismaterialandproducedshallowdonorlevels,turningthematerialintoanintrinsicn-typesemiconductor.10Theelectronsreleasedbythesedefectswerenotcompensatedbecauseacceptor-likeintrinsicdefectsconsistingofSnvoidsandOinterstitialsdidnotformspontaneously.Furthermore,thereleasedelectronsdidnotmakedirectopticaltransitionsinthevisiblerangeduetothelargegapbetweentheFermilevelandtheenergylevelofthefirstunoccupiedstates.Thus,SnO2couldhaveacarrierdensitywithminoreffectsonitstransparency.10
Theconductivityσisintrinsicallylimitedfortworeasons.First,nandμcannotbeindependentlyincreasedforpracticalTCOswithrelativelyhighcarrierconcentrations.Athighconductingelectrondensity,carriertransportislimitedprimarilybyionizedimpurityscattering,i.e.,theCoulombinteractionsbetweenelectronsandthedopants.Higherdopingconcentrationreducescarriermobilitytoadegreethattheconductivityisnotincreased,anditdecreasestheopticaltransmissionatthenear-infrarededge.Within
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